The present invention relates to a measuring system and a method of detecting static electricity in a measuring object or test material as described more in detail in the preamble of the appended independent claims. The invention also relates to the use of the measuring system.
The above mentioned measuring system typically includes at least a sensor, a voltage meter and a processor unit with a signal processing unit connected to the voltage meter. The sensor of the measuring system comprises a sensing element, such as an antenna or an electrode, arranged so that the electric field of the measuring object or test material induces an electric signal in the sensing element indicating a change in electric charge which is proportional to the electric charge of the measuring object or test material. A detector is connected between the sensing element and the voltage meter. The detector consist typically of a capacitor circuit connected between the sensing element and the voltage meter, which transmits to the voltage meter the signal indicating a change in electric charge induced in the sensing element.
Simple reliable systems of this type suitable also for industrial processes have not as yet been available on the market.
A measuring system for measurement of electric charge in turbulent material flowing through a pipe, so as to find out the risk of explosions in the material, has been suggested in U.S. Pat. No. 3,753,102. Variations in the electric charge of the turbulent material which flows past an electrode are measured with an AC voltage meter. The AC component induced by the changes in the electric field is conducted directly via an AC amplifier and a rectifier to a DC voltage meter. The measuring system seems to be very sensitive, and requires protection against interference from external electric fields.
U.S. Pat. Nos. 4,370,616 and 5,151,659 disclose similar measuring systems with direct measurement of electric charges, in this case induced in rapidly oscillating electrodes arranged in a protecting opening in front of the surface the charge of which is to be measured. The measuring signal is amplified in an amplifier before the voltage meter. Even this measuring system is sensitive to interference. The measuring signal and all interference signals, if any, are amplified and read off.
U.S. Pat. No. 4,716,371 and British Patent Specification No.1 568 811 disclose similar measuring systems with capacitive circuits. These measuring systems seem to be highly sensitive to interference.
It is an object of the present invention to provide a new, reliable and as to its design simple measuring system for detecting electric charges in e.g. flowing powdery mediums, gas flows, or material webs in various kinds of processes, in order to obtain measuring values which can be used for controlling these processes.
It is a particular object of this invention to provide a measuring system and a method which is also suited for use in industrial processes under severe, e.g. dust-laden and/or interference-laden conditions.
It is a further object of this invention to provide a measuring system having an amply sufficient sensitivity to signals of low frequencies, e.g. in the order of 1-10 Hz.
It is a further object of the invention to provide a measuring system in which the influence of interference can be minimized.
The above stated objects are achieved by means of measuring systems and methods which are characterized by what have been stated in the characterizing part of the appended independent claims.
A measuring system according to the invention includes in the detector a capacitive or capacitor circuit having an integrating effect on the measuring signals which are conducted via the detector from the sensing element, i.e. the antenna or electrode, to the voltage meter. The measuring values from the voltage meter are processed and evaluated in an processor unit.
According to a preferred embodiment of the invention a resistor R is connected in parallel to the capacitor circuit having the capacitance C, so that the time constant T=R*C of the capacitor circuit at discharging falls within the range of 0.01-5 s. The capacitor circuit will then have an integrating effect on the measuring signals (measuring values), which are conducted via the detector from the sensing element, i.e. the antenna or electrode, to the voltage meter, and are read by the voltage meter. The time constant for the recharging of the capacitor circuit is very small, negligible, because the resistance between the antenna and the capacitor is small. The time constant for the discharging is however made big by means of the resistor R, so that an integration of the measuring signals and a more even final measuring value is obtained. Thus, discrete signals are not measured, but integrated values of a larger number of signals. Integration of the measuring signals results in the fact that possible interference has less effect on the result. Interference at known frequencies can, if necessary, be filtered off.
In the measuring system according to the invention the resistance R in the detector circuit is chosen so as to be big, advantageously &gt;1 Mohm, typically between 1-100 Mohm, and preferably &gt;40 Mohm.
Also the capacitance has to be big, advantageously &gt;1 nF, typically between 1-50 nF, and preferably 4-12 nF.
The time needed for the voltage U to be discharged to 63.2% over the capacitor C is called the time constant T=R*C of the capacitor circuit. In a measuring system according to the invention, the resistance R and the capacitance C for the capacitor circuit are chosen so that the time constant falls within the range of 0.01-5 s, typically within the range of 0.1-1 s, and preferably within the range of 0.5-0.8 s. In a capacitor circuit having a big time constant, the voltage has not time to drop to the starting-point between two subsequent impulse-like measuring signals. This means that the capacitor circuit has an integrating effect and that all measuring values in this system are measured on a more even level in comparison with prior art measuring systems, which do not have a corresponding capacitor circuit with a big time constant.
In the measuring system according to the invention the measuring value of the voltage follows very rapidly a change of dE/dt. The time constant at a change of dE/dt is only a few microseconds because the resistance between the antenna and the capacitor is very small. If on the other hand dE/dt goes rapidly towards zero, the voltage over the capacitor goes slowly towards zero. The time constant at discharging is thereby big, e.g. 0.5-1 s.
The capacitor circuit enables the measuring system according to the invention to take into consideration also measuring values at low frequencies in spite of the fact that the measuring system by means of the processing of the signals can be made less sensitive to occasional interference, e.g. to hyper charged field or external fields, such as fields from electric motors or the like.
The measuring system according to the invention is a computerized measuring system for measurement of strength, changes and polarity of electric fields caused by electrostatically charged materials. A sensing element, here also called antenna, is arranged so that the electric field induces a charge concentration in the antenna. The antenna does not have to be in direct contact with the material the charge of which is to be measured. The charge can, depending on the design of the antenna and the magnitude of the charge, be measured even at a distance of 50 cm, preferably 20 cm. The magnitude of the measuring signal is proportional to the strength of the electric field. The functional principle is based on changes in the electric field and requires therefore a relative movement between the antenna and the measuring object.